It can help scientists located regions most susceptible to melting.
Greenland is home to over 60 glaciers, all of which are part of the Greenland Ice Sheet. Unfortunately, the ice sheet has been shrinking year after year. In fact, just last month, one-tenth of the ice sheet’s surface was melting due to an unseasonably warm spring, smashing the 2010 record for the earliest thaw.
Since 99 percent of the planet’s freshwater ice is locked up in the Antarctic and Greenland ice caps, monitoring melting rates is a priority. If these ice sheets melt, sea levels could rise by 6 feet this century, and far higher in the next.
With that in mind, researchers from MIT and Princeton University developed a new technique to monitor seasonal changes in Greenland’s ice sheet — using seismic vibrations produced by crashing ocean waves. The results, which will be published in the journal Science Advances, may help scientists locate regions that are most susceptible to melting.
"One of the major contributors to sea level rise will be changes to the ice sheets," said Germán Prieto, Assistant Professor in the Department of Earth, Atmospheric and Planetary Sciences (EAPS) at MIT, in a press release.
Seismic waves generated by the ocean propagate through Earth’s crust at speeds that depend, in part, on the crust’s porosity. The more porous the rocks are, the slower the seismic waves travel. Aurélien Mordret, a postdoc at EAPS, hypothesized that the speed of seismic waves could reflect the volume of ice lying above.
Any overlying mass, like an ice sheet, acts like a weight on a sponge — squeezing the pores closed or letting them reopen, depending on whether the ice above is growing or shrinking in size. "By looking at velocity changes, we can make predictions of the volume change of the ice sheet mass," Prieto said.
Changes in ice sheets are usually tracked using laser altimetry, where airplanes fly over a region and send a laser pulse down and back to measure ice sheet topography, and using data from NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites. However, "you can only do laser altimetry several times a year, and GRACE satellites require about one month to cover the Earth's surface," explained Prieto.
But ocean waves and the seismic activity they produce signals that sensors can pick up continuously, day by day. "This has very good time resolution, so it can look at melting over short time periods, like summer to winter," Prieto said.
To test this, the researchers collected seismic data from a small sensor network on the western side of the Greenland ice sheet from January 2012 to January 2014. Although these sensors monitor glaciers and earthquakes, Prieto’s team is the first to use the data to monitor the entire ice sheet.
Looking through the data, the team was able to detect small changes in the velocity of seismic waves, with large seismic decreases in 2012. These measurements matched the observations of ice sheet volume made by the GRACE satellites, which recorded abnormally large melting in 2012.
So seismic data may reflect changes in ice sheets.
Next, the team plans to use available seismic networks to track the seasonal changes in the Antarctic ice sheet. If the technique is proven reliable in Antarctica, Prieto hopes to start a large-scale project involving seismic sensors being distributed along the coasts of Greenland and Antarctica.
This technique could also help scientists better understand how ice sheets contribute to changes in sea levels.
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